1. Field of the Invention
This invention relates to the simultaneous or sequential introduction of multiphase matter into a plastic melt through a plasticating screw.
2. Prior Art
Heretofore, plastic in molds or dies have been treated by introduction of matter through the wall of the mold, for cooling or the like, or matter has been added only to the plastic as it is worked between the lasticating screw and the housing in which it is rotatively supported.
The present invention discloses a novel way to introduce high pressure gas, vapors and/or liquids into the process of manufacturing plastic material by a plasticating screw machine. The invention comprises drilling (coring) a hole axially through a plasticating screw of appropriate size to accommodate the introduction of gas, vapors and/or liquids into the melt shot.
A non return shut off valve may be arranged on the tip of that plasticating screw, which valve is made with a drilled hole through it including the tip so that during or after the injection phase (short shot) or in the process of completion the tip opens either due to air pressure, mechanical actuation (spring loaded) etc. to allow the assist gas, vapors or liquids to hollow out the plastic part, where that screw is utilized in an injection molding operation. A finely controlled gas (typically nitrogen) pressure and volume may be applied to the driving (proximal) end of the plasticating screw by means of a rotational collar. The gas will pass through the length of the screw to the non return shut off valve, or tip, where it will enter the molten plastic material. Required gas volume or material viscosity may necessitate fine gas entry apertures such as with a jewelled orifice and/or a gas check valve to prevent material from blocking gas passages. A typical processing sequence will be incorporated to the gas cycle. The part will be a short shot. The screw will remain fully forward through the second stage while gas is applied (no cushion). After this phase, allowances must be made for venting or degassing of the unnecessary gas before the part is ejected, when utilized in an injection molding format. The introduction of gas, vapors, or liquids into the plasticating screw is from the rear of the screw in one embodiment, or from a design mated entry point, such as a radial channel from a collar radially outwardly of the screw and into the core of the reciprocating injection screw ina further embodiment. Assist media is proportionally regulated via the described gas control system to allow for maximum efficiency of profiling.
A description of the mechanical aspects of the invention includes: (a) Injection screw coring for the introduction of gas, vapor, or liquids (fluids); (b) Non return shut off valve threaded onto the delivery end of the plasaticating screw for the purpose of: (1) shuting off the backflow of the molten plastic during an injection, holding and cooling phase of the plastic, (2) Enhancing the mixing of the molten plastic, (3) Allowing for the passage of high pressure gas, vapors or liquids through the center hole drilled (to specific size) through the center of the injection screw tip (aka non return valve), (4) Prevent plastic coming into center core during the screw recovery or plastification phase.
The center bore in the plasatication screw is arranged to accommodate the passage of gas, vapor, liquids through the screw injection tip under certain pressures and temperatures. The center core hole should also be able to accommodate single, double or multiphase coaxial tubes for the passage of multiphase materials with a conveying point in the injection tip. Temperature control is of paramount value for the constituent plastics being worked.
A high pressure rotary union may be arranged at the back end of the screw depending as to whether constituents are introduced axially or radially.
The phase process for injection molding plastics utilizing the center cored screw comprises: (a) Injection phase: control by a control circuit of the melt and high pressure gas, vapor, or liquid and temperature control thereof, (b) Holding and Cooling phase: control of gas, vapor, or liquid pressure and temperature for efficient cooling of the molded parts to control surface finish and part shrinkage. This is accomplished by: (1.) Introduction of gas, vapor, or liquid simultaneously (without a time delay) using control circuit components comprising a further embodiment of the present invention; or (2.) Introduction of gas, vapor, or liquid sequentially (after a preprogrammed time delay) the control circuit components of the present invention; (3.) Temperature control of the gas to provide an optimal xe2x80x9cgas bubblexe2x80x9d, coring, or tunneling, or thereof into the plastic melt; and (4.) Profiling of gas pressure by use of software with a programmed capability to ramp or step up the gas pressure using a menu driven system allowing various time/pressure durations in the gas injection phase.
A PID (proportional integral derivative) controlled micro-controller, comprises a further embodiment of this invention. This Proportional Controller is a closed loop PID controlled pressure regulating system, having a digital (LCD) display used as an attachment to commercially available gas control systems.
The proportional control is achieved through an electro-pneumatic circuit with a proportional regulator (or valve) pilot operating a booster unit which then pilot operates a high pressure regulator. Low pressure compressed air 0.55-0.70 MPa (typically 80-150 psi) is used as a control pressure which then regulates the pressurized gas to desired levels up to a maximum of 103 MPa (xcx9c15,000 psi max.). With this system, as the low pressure signal is boosted to the respective high pressure setting, it xe2x80x9cloadsxe2x80x9d a xe2x80x9cdome loadedxe2x80x9d high pressure regulator. The proportional regulation is controlled by a Pulse Width Modulating (PWM) controller operating a normally closed on/off three way valve. Upon the completion of the cycle, the line pressure from the gas tubing can be vented through a vent port in the xe2x80x9cdomexe2x80x9d regulator.
A downstream high pressure transducer attached to the gas line provides feedback to the micro-controller thus adjusting the PWM proportional valve to the set point (closed loop). This closed loop system with a full PID control algorithm and controlled by the micro-controller allows the gas pressure to be controlled via a voltage (or current) profile thus creating an infinite number of pressure versus time settings. An on-board RS232 interface allows the capability to capture the information from the gas pressure transducer before the gas enters the mold and thus may be used as a monitoring and statistical tool. A PC displays a menu system, where pressure can be profiled either up or down as a geometric function.
A digital display is arranged to show the pressure set point and the actual value (during cycle). The micro-controller (PWM) is driven using a separate 0-24 VDC, 4.0 amps max. rated regulated power supply. Ports for the pressure sensor, RS232, allow for data acquisition and valve control signals (#1, #3, #5, #6) allow voltage input into the controller board. Using the up/down keys, the pressure settings are easily adjusted within a pressure sensitivity scale of +xe2x88x921 psi and can be seen on screen as setpoint vs. actual. Adjustments to the software are made using diagnostics modeand coarse adjust. The response time of the PID controller from the start of the cycle to reaching the setpoint is approximately 50 milliseconds (response time can be adjusted to specific needs).
A prototype of the micro-controller is currently used with the gas control system on for example, a 250 Ton 32 oz. Cincinnati Milacron injection molding machine as an added feature to study the process parameters of the gas-assisted injection molding process.
A Gas-Assist Controller system will be attached to an existing commercial gas unit utilizing the volume controll technique and will be operational as an integrated attachment. Tne system has features for multiple independent channels (if needed), closed loop control, 0-5 VDC control signals, PID control algorithm, pulse width modulating (PWM) outputs, option for extra pressure transducers, input signal conditioning, RS232 monitor, and operator interface (Keyboard/LCD).
Equipment operation for gas heating/cooling by this system would allow for pressurization of N2 by the gas (N2) itself or shop air (150 psi), whichever suits per need.
If N2 is not used as a control mechanism, a Tee fitting and gas regulator would be eliminated from the overall system operation. A high pressure gas (up to 15,000 psi) may be used as both the control mechanism as well as the actual set pressure gas to hollow out the molten plastic. A proportional regulator is arranged to provide precise pressure control with a flow control valve loading a xe2x80x9cDomexe2x80x9d loaded high pressure regulator to make the gas setpoint (set on the PID controller). A second Tee fitting may be used as a split for the gas injection stage and the gas packing/holding/cooling state.
During the gas injection stage, once the gas has reached the setpoint at the xe2x80x9cdomexe2x80x9d regulator, a first circuit would allow the actuation of a 2 way solenoid valve. At this point, a heat exchanger heating element of the system would heat the gas to a desired set temperature and fill the gas into the accumulator. At this point, both the pressure and the temperature would be measured by a pressure transducer and a temperature sensor. Next, a solenoid valve is arranged in the system to open and allow the free passage of pressurized gas through a fixed jewel orifice and into the mold cavity. A pressure sensor and a temperature sensor may be placed before the jeweled orifice to measure the pressure and temperature just before the gas enter the mold.
Upon the completion of gas injection cycle, the solenoid valves are arranged to close. The N2 would now be diverted to a second circuit where a humidifier device may add small amounts of H2O to the N2 and the components would be cooled to a set temperature by a heat exchanger cooling element in the system. Again, a 2 way solenoid valve is arranged to open, allowing the mixture to pass through the orifice with pressure sensor and temperature sensor recording the pressure and temperature of the cold mixture. This phase of the cycle would assist in making hollow injected molded parts.
Thus the invention includes: (1) a gas injection molding system comprising a screw for delivering plastic material to a mold and a gas injection jeweled orifice port through the screw tip for injecting gas, vapor, or liquids into the plastic in the mold; (2) a gas injection closed loop pressure control system using a dome regulator ratio device with a micro-processor based control system to profile (ramp/step) the gas injection using a menu driven system allowing various time/pressure durations in the gas injection phase and the gas holding/packing phase. This can be either a personal computer (PC), Programmable Logic controller (PLC) or an embedded system with either a Proportional Integral Derivative (PID) or FUZZY logic algorithm; and (3) a control system for controlling the temperature of the gas during the injection phase and cooling phase.
The invention further includes a control for the injection stage velocity of the gas. A controlled pressure in a tank is arranged to deliver a controlled velocity through a fixed jeweled orifice. Due to the sonic flow of gas [If, P1+1.013 less than =1.896 (P2+1.013) then the flow rate is dependent only on the orifice size and supply pressure, P1].
The temperature of the gas may be heated by a heater. This will increase the gas viscosity thus limiting or helping prevent xe2x80x9cgas blow outxe2x80x9d from the molded part during the process cycle. It will also add to the temperature uniformity which will decrease the tendency of the part to have sink marks due to the thermal differences between the melt, mold and gas.
Once the mold has been filled and the part has been properly cored out with gas, pressure control is necessary to keep the part xe2x80x9cpackedxe2x80x9d and reduce the sink effect. This will be accomplished by switching to the second portion (part B) of circuit which regulates pressure only. This will allow for pressure profiling in the holding/packing phase depending upon the part geometry and material. Simultaneously we will be introducing water (H2O) to the nitrogen (N2) mixture. This will enter the hollow core of the part thus providing a faster cooling phase, leading to a decrease in cycle time. The after cooler may not be necessary as the effect of phase change of liquid to steam of the H2O may remove sufficient heat.
It is proposed to inject other gases into the plastic hollow part once it has expanded. A number of gases can be used. One such effective way is to have a phase change in the part for doing the cooling. A mixture of cold air with water spray will convert the spray into ice crystals which are conveyed with the air and once they hit the inside of the plastic part the ice will convert the water vapor with a large latent heat of sublimation (about 1100 Btu/lb). As the ice sublimates, the pressure will increase and a control device will allow the part to be vented via a pressure regulator after which the process of air/ice injection is repeated until the part is sufficiently cooled.
Alternatively one can inject liquid CO2 ( greater than =80 psi). The liquid turns to dry ice at P less than =80 psi and then vaporizes (sublimates) when it contacts the internal wall of the part. A third method would be to use liquid nitrogen. The liquid would evaporate and extract heat from the inside of the part. Also, the nitrogen (N2) could be recovered.
In the present invention the hydraulic injection cylinder is coupled to the screw shank for inject and retract functions. The screw shank may also pass through a gearbox which provides rotation during plasticizing. The flight includes feed, transition, and metering sections of the screw. The screw tip or valve will typically incorporate a check ring or ball.